Usually the problem is in config.php ... for example :
- there should be no last php end tags ( ?> ) in any php files of moodle 2.X and particularly not in config.php
- config.php may be in a wrong file format, it should be saved as an ANSI file, not UTF-8 or UTF-8 with BOM ... some text editors may change the format when you edit and save config.php
- config.php may have whitespaces or some extra code before the first php tags
- you may need to purge all caches either from administration menu of moodle or from moodledata/cache folder
Check also that paths in config.php are correct and that you have given the permissions for actual IIS_USER. You might find some "real"URL of non-existing image if you use mouse right click - Properties over image place holder in IE if opening in a new tab leads to 404 page
And if none of this helps the reason might be in some server side setting or url rewrite script but it's much more difficult to quess it without access to your server - or source code (in your browser you can view the source and for example yui css could be broken by some minifier scripts etc)